1. Field of the Invention
This inventlon relates generally to printed circuit boards, and particularly to such a printed circuit board with which solderability can be checked easily by observing a checking pattern provided thereon.
2. Description of the Prior Art
There are various ways of preventing solder bridgings from occuring between land portions on a printed circuit board, when a printed circuit board having electronic parts mounted thereon is dipped in a solder bath for soldering lead terminals of the electronic parts with the land portions on the printed circuit board.
FIG. 1 is a top plan view of a printed circuit board of a prior art whose land portions are arranged to prevent solder bridging from occuring (Examined Japanese Patent Publication No. 58-2470), and FIG. 2 is an enlarged cross-sectional view along a line V--V in FIG. 1 showing that an electronic part is soldered on the printed circuit board.
In FIGS. 1 and 2, reference numeral 21 designates a printed circuit board which includes an insulative substrate 22, an electroconductive layer 23, and a resist layer 26. The electroconductive layer 23 is coated on the insulative substrate 22. The resist layer 26 is coated on the electroconductive layer 23 in such a manner that a short land portion 24A and a long land portion 24B are formed adjacent to each other. Further, a through-hole or opening 25 is provided at a central portion of each of the land portions 24A and 24B. The through-holes 25 penetrate the electroconductive layer 23 and the insulative substrate 22 so that a lead terminal pin 28 of an electronic device e.q. an integrated circuit (IC) 27 is inserted into the through-holes 25. Provided that the length from the through-hole 25 of the short land portion 24A to its one end is l.sub.1 and the length from the through-hole 25 of the long land portion 24B to its one end is l.sub.2, the relation between l.sub.1 and l.sub.2 are selected to satisfy, 2l.sub.1 .ltoreq.l.sub.2 .ltoreq.3l.sub.1. Accordingly, the line drawn by connecting one end of the land portions 24A and 24B makes a saw tooth line in the direction of an arrow A.
When the printed circuit board of FIG. 1 is moved in the direction of an arrow B and dipped in a solder bath (not shown) to solder the lead terminal pins 28 of the IC 27 to the through-holes 25 at the land portions 24A and 24B, each of the land portions 24A and 24B departs from the surface of melted solder at different timings, so that solder bridging does not occure between the land portions 24A and 24B which are adjacent to each other in the direction of the arrow A.
Although the solder bridging does not occure between the land portions 24A and 24B on the printed circuit board 21 by carrying out the soldering in the above described manner, it is very difficult to examine wettability of the solder for performing a good soldering without solder bridging. Therefore, in this printed circuit board the wettability was adjusted on the basis of an operator's experience, knowledge and so on by observing a conditon of a solder 29. Thus, the soldering condition, that is, the wettability of the solder differs depending upon the judgement of an operator. It may therefore be supposed that the lead terminal pins 28 of the IC 27 are soldered on the land portions 24A and 24B differently, as shown in FIGS. 3A, 3B, and 3C, dependent upon the different wettability of solder.
FIG. 3A shows a case where the wettability of solder is small. In this case, cavity 29A is formed around the terminal pin 29 and on the layer 23 inside the solder 29. Connection between the solder 29 and the terminal pin 28, and the layer 23 seems to be in a good condition for a while after soldering, however, the solder 29 may come off by secular variation.
FIG. 3B shows a case where the wettability of solder is adequate or optimum, and the soldering is satisfactorily made.
FIG. 3C shows a case where the wettability of solder is large. Thus, connection of the solder 29 to the lead terminal 28 and the layer 23 is made with a small amount of solder, so that the soldering strength is not enough.
It has therefore been desired to provide a printed circuit board by which the wettability of solder can be checked in a simple manner since the wettability is a parameter to determine whether or not the soldering can be satisfactorily made.